Orthopaedic prostheses greatly improve the quality of life for many individuals that suffer bone loss due to disease or a congenital abnormality. The success of these devices is dependent upon the body's ability to synthesize new bone in direct apposition to the implant surface. In consequence, considerable research effort has been directed at understanding the factors that promote the association of bone cells with implants. This association, like most cell/matrix interactions, is thought to be directed by the integrin family of cell adhesion receptors. In tandem with mediating cell attachment, integrins appear to play a role in regulating the differentiation of bone precursor cells, an event that is prerequisite for new bone synthesis. Studies of prostheses that have been retrieved from laboratory animals suggest that devices coated with synthetic bone analogue, hydroxyapatite, have substantially more bony in-growth than uncoated metal implants. A central hypothesis is therefore proposed that hydroxyapatite is superior to titanium to promoting integrin-mediated osteoblast attachment and differentiation. Preliminary data from our laboratory reveal that purified integrins do, indeed, bind better to serum-coated hydroxyapatite than to serum-coated titanium. Moreover, integrin ligands within serum adsorb preferentially to hydroxyapatite, thus enhanced integrin binding is likely due to the increased concentration of ligand at the hydroxyapatite surface. Finally, osteoblast precursor cells, similar to purified integrins, demonstrate markedly better binding to hydroxyapatite than to precursors and implant biomaterials. As part of Specific Aim 1, we will identify the specific integrins and integrin will examine the events downstream of integrin activation, such as tyrosine kinase-mediated signal transduction, cytoskeletal reorganization and extracellular matrix assembly. Finally, in Specific Aim 3, osteoblasts grown on either hydroxyapatite or titanium will be evaluated or differentiation, as measured by the expression of differentiation-specific proteins, as well as by mineralization assays. Collectively these experiments will provide important insight into the cellular and molecular mechanisms that contribute to the enhanced bone- ingrowth that has been observed with hydroxyapatite-coated implants, relative to uncoated metal devices.